Stomatal Movements in Plant Cells

The following points highlight the four factors affecting stomatal movements in plant cells. The factors are: 1. Light 2. Carbon Dioxide Concentration 3. Temperature and 4. Water Deficits and Abscisic Acid.

Factor # 1. Light:

Light has strong controlling influence on the stomatal movements. Stomata generally open in light and close in darkness. The amount of light required to achieve maximal stomatal openings varies with the spe­cies. For instance, some plants like tobacco require low light intensities (as low as 2.5% of full daylight) while others may require lull sunlight.

The stomata of plants showing CAM (Crassulacean Acid Metabolism) are exceptional because they open at night and close during the day. These plants absorb CO₂ and fix it into organic acids at night. During the day time, CO₂ is released from the organic acids and is reduced photo-synthetically.

According to Kramer (1959), the stomata in some plants may be induced to open by bright moonlight. The duration during which stomata remain open in daylight and close at night varies among plant species. The effect of different wavelengths of light on stomatal opening also varies. For instance, Zelitch (1963) did not observe stomatal opening in tobacco leaves which were exposed to far- red or ultra-violet irradiations. Green light was also ineffective in stomatal opening.

The action spectrum of the effect of light on stomata bears a resemblance with that of photosynthesis with a blue light effect superimposed. Some plants lack the photosynthetic spectrum and are sensitive to blue light only. The photosynthetic component may be due to photosynthesis in guard cells which contain chloroplasts.

Thus, light may have controlling influence on stomatal opening in the following ways:

(i) Photosynthesis reduces the CO₂ conc. in guard cells which has powerful stimulus for opening the stomata.

(ii) Osmotically active substances such as soluble sugars are synthesised during photo­synthesis which may contribute in decreasing the water potential of guard cells.

(iii) Photophosphorylation may provide ATP which is required to operate H⁺/K⁺ ions exchange pumps in which H⁺ ions are pumped out and K⁺ ions, CI^– ions and organic anions are pumped into the guard cells. These ions decrease water potential of guard cells.

(iv) In many plants illumination of the guard cells results in an increase of pH (which may be a consequence of reduced conc. of CO₂ in guard cells). High pH favours hydrolysis of starch into osmotically active sugars (glucose-1-phosphate) In the presence of enzyme starch phosphorylase thereby decreasing the water potential of guard cells. (Reverse reaction occurs in dark when pH is low). However, this starch ⇋ sugars inter-conversion hypothesis is not widely accepted because (a) it is not universally true and (b) very small amounts of osmoti­cally active or soluble sugars have been extracted from the guard cells.

(v) Low conc. of CO₂ and high pH (pH 7) in guard cells in daylight may favour synthesis of malic acid from HCO^–₃ + Phosphoenol pyruvic acid (PEP) in the presence of the enzyme. PEP-carboxylase. Malic acid would produce protons (H⁺) which can operate in ATP-driven H⁺/K⁺ ions exchange pumps.

The blue light component of action spectrum on the effect of light on stomatal opening is related to a different photoactive control through cryptochrome (most probably through the carotenoid zeaxanthin).

According to one such scheme, blue light is perceived by zeaxanthin in the chloroplast of guard cells.

The excitation of zeaxanthin by blue light then starts signal transduction process that in­cludes,

(i) Isomerization of zeaxanthin,

(ii) Conformational changes of an apoprotein,

(iii) Transmis­sion of blue light signal across the chloroplast membrane by a second messenger (most probably Ca⁺⁺, phosphatases, calcium binding protein, calmodulin and inositol triphosphate i.e., IP₃), (iv) activation of H⁺ – ATPase at the guard cell plasma membrane resulting in pumping of protons across the membrane and intake of K⁺ ions. Blue light also stimulates degradation of starch and synthesis of malate. Accmulation of solutes in guard cells ultimately leads to stomatal opening.

Factor # 2. Carbon Dioxide Concentration:

Concentration of CO₂ has pronounced effect on stomatal movement. Reduced CO₂ conc. favours opening of stomata while an increase in CO₂ conc. promotes stomatal closing. Under experimental conditions, the stomata can be induced to open even in dark if conc. of CO₂ is significantly lowered below that of normal air. On the other hand, a marked increase in CO₂ conc. above that of normal air causes the stomata to close in dark as well as in light.

The stomata which are forced to close by high CO₂ conc., do not reopen rapidly simply by flushing the leaf with CO₂ free air and in dark. However, during subsequent light exposure such stomata open soon. It is because CO₂ trapped inside the leaf is consumed in photosynthesis during light exposure.

Therefore, it is in-fact that CO₂ which is present inside the leaf (intercellular) rather than that of the outer atmosphere which has controlling influence on sto­matal movement. The cuticle present over the guard cells and epidermal cells is quite imperme­able to CO₂ and ensures response of stomata to CO₂ present in the leaf rather than that of the outer atmosphere.

Factor # 3. Temperature:

Usually an increase in temperature results in increased stomatal opening provided water does not become a limiting factor. Stomata of some plants, e.g., Camellia do not open at very low temperatures (below 0°C) even in strong light. On the other hand in some plants the sto­mata tend to close even at high temperatures (more than 30°C). This may be due to increased CO₂ conc. inside the leaves caused by increased respiration rate at high temp, and heat-impaired photosynthesis in the latter category of plants.

Factor # 4. Water Deficits and Abscisic Acid (ABA):

When rate of transpiration exceeds the rate of absorption of water, a water deficit is created in plants. Such plants begin to show sings of wilting and are known as water-stressed plants. Most of the mesophytes under such conditions close their stomata quite tightly and completely in order to protect them from the damage which may result due to extreme water shortage. The stomata reopen only when water potential of these plants is restored. This type of control of stomatal movement by water is called as hydro passive control.

Accumulation of phytohormone abscisic acid (ABA) in the guard cells of many different water-stressed plants is now well established. The ABA causes stomata of such plants to close. When water potential of the water-stressed plant is restored, the stomata reopen and ABA gradually disappears from the guard cells. This type of control of stomata by water (mediated through ABA) has been called as hydro active control. Externally applied ABA to leaves of normal plants is also known to induce closure of stomata and the idea is growing that ABA is a primary regulator of stomatal action in water-stressed plants.